The present invention is related to an arrangement of micromechanically manufactured ultrasound transducers for emitting ultrasound in fluids or in biological tissue.
Micromechanically manufactured ultrasound transducers are utilized for emission of ultrasound into fluids or into biological tissue. Such ultrasound transducers can be utilized individually or in an arrangement of a plurality of individual transducers. Each ultrasound transducer is composed of a micromechanical structure that comprises a membrane that is electrically excited in some way or other and emits ultrasound. Given an arrangement of ultrasound transducers, the area of the arrangement is large compared to the wavelength of the generated ultrasound. The membranes are small in diameter and much thinner than the wavelength of the emitted sound. Due to the extremely slight weight of the oscillating masses, i.e. the membranes, a micromechanical ultrasound transducer is in the position to emit and to receive short sound pulses with good efficiency.
What is disadvantageous about this arrangement is that the membranes reverberate a long time after a pulse and thereby emit slight sound. These oscillations are produced by waves that planarly propagate, i.e. not in the normal emission direction perpendicular to the membrane but in the planar expanse of an ultrasound arrangement or array of ultrasound transducers. Due to such decay oscillations of an arrangement of ultrasound transducers, the transmission and reception behavior is thus negatively influenced. Such oscillations are extremely unbeneficial since the frequency of the noise signal thereby emitted lies below the center frequency of the useful signal. The noise signal thus propagates better in the working medium than does the useful signal. Further, the decay oscillation of the transducer disturbs the reception of the sound pulse reflected from the subject under test given pulse-echo operation.
Conventional ultrasound transducers have larger oscillating masses and therefore the aforenoted problems do not exist. For generating short pulses, the useful oscillations and possible noise oscillations are attenuated to the same extent. The attenuation usually occurs by impedance-matched damping compounds at the backside (backing) or by inner attenuation of the thickness oscillator.
The invention is based on an object of damping noise signals occurring within an arrangement of ultrasound transducers that propagate in the direction of the planar expanse of the arrangement.
In an embodiment, an array of micromechanical ultrasound transducers is provided which comprises a plurality of transducers. The array has a front side. Each transducer comprises a membrane that is excited according to an electro-mechanical principle. The front side of the array comprises a damping layer that comprises a polymer material. An operating temperature of the array, given a predetermined operating frequency, is in the glass transition range of the polymer material.
In an embodiment, the damping layer has a thickness such that a corresponding Eigen-frequency of the damping layer corresponds to the operating frequency of the transducers.
In an embodiment, the thickness of the damping layer ranges from 10 to 50 xcexcm.
In an embodiment, the polymer material is an elastomer.
In an embodiment, the elastomer is polyurethane or silicone.
In an embodiment, the transducers are arranged in the form of a rectangular matrix.
In an embodiment, the transducers are arranged in the form of a hexagonal matrix.
In an embodiment, the transducers are arranged in the form of a circular matrix.
In an embodiment, the transducers work according to an electrostatic principle and the membranes represent one of two capacitor electrodes.
In an embodiment, the transducers work according to a piezo-electric principle and the membranes represent a piezo-electric layer.
The damping of a membrane given a micromechanically manufactured ultrasound transducer cannot occur at the backside of the membrane since this is not freely accessible. Micromechanical ultrasound transducers are usually constructed in a hard carrier material, for example silicon, so that no damping parts are to be anticipated proceeding from this side. The invention is based on the perception that the membrane damping is possible from the front side on the basis of a layer of polymer material covering the entire field of the ultrasound transducer arrangement. The operating temperature of the ultrasound transducer system preferably lies in the temperature range of the glass transition temperature of the polymer material, whereby the temperature range of the glass transition is dependent on the operating frequency. Operating frequency and operating temperature are to be considered in common in order to determine the suitable polymer material, since the average temperature of the glass transition increases with the operating frequency. The glass transition range represents the temperature range wherein the polymer material converts from a solid into a soft state. In this state, the material has especially high shearing attenuation and a moderate compression attenuation. It is assured as a result thereof that a slight attenuation occurs in emission direction and an especially high shearing attenuation is present transversely relative to the emission direction.
It is advantageous given especially high operating frequencies in the megahertz range (MHZ) and given operating temperatures in the proximity of room temperature, to utilize an elastomer as polymer material.
For maximum attenuation of the undesired oscillations, the layer thickness of the polymer material is to be selected such that the membrane oscillation is in resonance with an oscillation of the coating at the operating frequency. This coating resonance is not a matter of a thickness oscillation as in the classic N4 adaptation. On the contrary, the coating oscillates parallel to the transducer surface between the membranes and the membrane interspaces. A coating manufactured according to these criteria deteriorates the amplitude and the duration of the useful signal only slightly, but effects an effective attenuation of the noise oscillations in lateral direction. It is especially advantageous to employ an elastomer such as polyurethane or silicone as damping layer. These materials have the required properties in order to damp noise oscillations between different ultrasound transducers of an arrangement.
Both capacitative ultrasound transducers as well as those that work according to the piezo-electrical principle can be damped.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.